To determine the mass flow rate a fluid flowing in a pipe and particularly of a liquid, use is frequently made of Coriolis mass flowmeters, which, as is well known, induce Coriolis forces in the fluid and derive therefrom a measurement signal representative of mass flow rate by means of a vibratory transducer and control and evaluation electronics connected thereto.
Such Coriolis mass flowmeters have been known and in industrial use for a long time. For example, U.S. Pat. Nos. 4,756,198, 4,801,897, 5,048,350, 5,301,557, 5,349,872, 5,394,758, 5,796,011, and 6,138,517 as well as EP-A 803 713 disclose Coriolis mass flowmeters incorporating a transducer which comprises:                a double flow tube configuration communicating with the pipe and comprising        a first flow tube, which vibrates in operation, and        a second flow tube, which vibrates in operation,                    the first and second flow tubes vibrating in phase opposition;                        a vibration exciter for driving the flow tubes; and        vibration sensors for detecting inlet-side and outlet-side vibrations of the flow tubes and for producing at least one electric sensor signal influenced by the mass flow rate,        the vibration exciter and/or the vibration sensors having at least one magnetic circuit arrangement for converting electric into mechanical energy and/or vice versa which comprises:        at least one coil which is traversed at least temporarily by a current and penetrated at least temporarily by a magnetic field;        a first armature, fixed to the first vibrating flow tube of the transducer;        a second armature, fixed to the second vibrating flow tube of the transducer; and        a holder for the coil.        
As is well known, bent or straight flow tubes of such transducers, if excited in the so-called useful mode into flexural vibrations according to a first natural vibration mode shape, can cause Coriolis forces in the fluid passing therethrough. These, in turn, result in coplanar flexural vibrations being superimposed on the excited flexural vibrations of the useful mode in the so-called Coriolis mode, so that the vibrations detected by the vibration sensors at the inlet and outlet ends have a measurable phase difference, which is also dependent on the mass flow rate of the fluid.
In operation, the flow tubes of the transducer are usually excited at an instantaneous resonance frequency of the first natural vibration mode, particularly with the vibration amplitude maintained constant. As this resonance frequency is also dependent on the instantaneous density of the fluid in particular, commercially available Coriolis mass flowmeters can also be used to measure the density of moving fluids.
In magnetic circuit arrangements as disclosed in U.S. Pat. No. 5,048,350, both the armature and the associated coil are fixed directly to the double flow tube configuration, so that in operation, both, following the motions of the associated flow tubes, are practically permanently accelerated. The resulting inertial forces, which affect particularly the coil, may lie in ranges far above 10 G (=weight). Even inertial forces up to 30 G are nothing unusual. Because of these high mechanical stresses, the coils in such magnetic circuit arrangements, and particularly their windings, must be highly loadable to ensure a long life of the vibration exciters, particularly a high number of vibration cycles, with unchanged accuracy in operation.
In magnetic circuit arrangements as disclosed in U.S. Pat. Nos. 4,756,198, 5,349,872, or 6,138,517, for example, such mechanical stress on coils is avoided by holding each of the latter in a holding structure that is at rest relative to the vibrating flow tubes, such as a support plate, a meter housing, or a support frame flexibly attached directly to the flow tubes, at a nearly constant distance from a centroidal axis, here a vertical axis, of the double flow tube arrangement.
It turned out, however, that although the above-described mechanical stresses can thus be virtually completely eliminated, the accuracy of such a magnetic circuit arrangement may be seriously affected by, particularly temperature-induced, shifts between the holding structure and the double flow tube configuration as occur, for example, in applications for fluids with widely varying temperatures. Because of the resulting different expansion of the holding structure and the double flow t tube configuration, which are neutralizable only limitedly, the rest positions of armature and coil change relative to each other.
While in the magnetic circuit arrangement according to U.S. Pat. No. 6,138,517, mainly a very great temperature difference, and thus a very great expansion difference, may occur between holding structure and double flow tube configuration, in the magnetic circuit arrangements described in U.S. Pat. No. 5,349,872, whose magnetic fields, particularly in the areas of the armatures, are highly inhomogeneous, even slight disturbances may result in considerable inaccuracy. As a result, e.g., if the arrangement is used as a vibration sensor, the sensor signals may have a very poor signal-to-noise ratio and/or exhibit very high harmonic distortion. Furthermore, the magnetic field of a magnetic circuit arrangement as disclosed in U.S. Pat. No. 5,349,872 may act through a very large region, i.e., it may also penetrate adjacent components of the transducer and particularly other such magnetic circuit arrangements and/or the flow tubes with the fluid passing therethrough, thus inducing interference voltages, for example. Further disadvantages of such a magnetic circuit arrangement are discussed in detail in U.S. Pat. No. 6,138,517, for example.
To ensure high accuracy in operation despite those temperature-induced interfering effects on the aforementioned magnetic circuit arrangements, the large amount of technical complexity required in such mass flowmeters to compensate for temperature-dependent interferences has to be increased even further.
It is therefore an object of the invention to provide a magnetic circuit arrangement, particularly an arrangement for use in a fluid-measuring vibratory transducer, which has a long service life and particularly a high number of-vibration cycles, and which, particularly if the transducer is used for fluids with high and/or varying temperatures, has constantly high accuracy in operation. In addition, the magnetic circuit arrangement according to the invention is to be insensitive to extraneous magnetic fields.